Special circulations, Coronary, Pulmonary… Faisal I. Mohammed, MD,PhD

Objectives Describe the control of blood flow to different circulations (Skeletal muscles, pulmonary and coronary) Point out special hemodynamic characteristic pertinent to each circulation discussed

Blood Flow: Skeletal Muscle Regulation Muscle blood flow can increase tenfold or more during physical activity as vasodilation occurs Low levels of epinephrine bind to  receptors Cholinergic receptors are occupied Intense exercise or sympathetic nervous system activation result in high levels of epinephrine High levels of epinephrine bind to  receptors and cause vasoconstriction This is a protective response to prevent muscle oxygen demands from exceeding cardiac pumping ability

Exercise and Muscle Blood Flow

Muscle Blood Flow During Exercise Can ­ 20 fold during exercise. Muscle makes up a large portion of body mass Þ great effect on Cardiac output. Resting blood flow = 3 to 4 ml/min/100 gm muscle. Oxygen delivery can be increased by increasing the extraction ratio from 25% up t0 75% Capillary density ­’s markedly. Most blood flow occurs between contractions.

Local Regulation of Muscle Blood Flow during Exercise ¯ O2 during exercise affects vascular smooth muscle directly Þ vasodilation. Vasodilators (which ones?) 1. K+ 2. Adenosine 3. Osmolality 4. EDRF (nitric oxide)

Nervous Regulation Sympathetic release of norepinephrine (mainly a). Adrenals release epinephrine (b and a) norepinephrine (a + a little b). b receptors Þ vasodilation mainly in muscle and the liver. a receptors Þ vasoconstriction in kidney and gut.

Arteriole Resistance: Control of Local Blood Flow

Blood Flow: Heart Small vessel coronary circulation is influenced by: Aortic pressure The pumping activity of the ventricles During ventricular systole: Coronary vessels compress Myocardial blood flow ceases Stored myoglobin supplies sufficient oxygen During ventricular diastole, oxygen and nutrients are carried to the heart Extraction ratio is maximum (75%) during rest so an increase demand for oxygen means an increase blood flow

CORONARY CIRCULATION

Epicardial and Subendocardial Vasculature

Coronary bypass operation During this operation, the surgeon grafts segments of another vessel, usually a small vein from the leg, between the aorta and the coronary vessels, bypassing areas of blockage. Patients who require surgery often receive two to five bypasses in a single operation.

Angioplasty On the left, a plastic tube is inserted into the coronary artery until it reaches the clogged area. In the middle diagram, a metal tip with a balloon attached is pushed out the end of the plastic tube into the clogged area. On the right, when the balloon is inflated, the vessel opens. Sometimes metal coils or slotted tubes, called stents, are inserted to keep the vessel open.

Blood Flow: Brain Blood flow to the brain is constant, as neurons are intolerant of ischemia Metabolic controls – brain tissue is extremely sensitive to declines in pH, and increased carbon dioxide causes marked vasodilation Myogenic controls protect the brain from damaging changes in blood pressure Decreases in MAP cause cerebral vessels to dilate to insure adequate perfusion Increases in MAP cause cerebral vessels to constrict

Blood Flow: Brain The brain can regulate is own blood flow in certain circumstances, such as ischemia caused by a tumor The brain is vulnerable under extreme systemic pressure changes MAP below 60mm Hg can cause syncope (fainting) MAP above 160 can result in cerebral edema

Blood Flow: Skin Blood flow through the skin: Supplies nutrients to cells in response to oxygen need Aids in body temperature regulation and provides a blood reservoir Blood flow to venous plexuses below the skin surface: Varies from 50 ml/min to 2500 ml/min, depending upon body temperature Is controlled by sympathetic nervous system reflexes initiated by temperature receptors and the central nervous system

Characteristics of the Pulmonary Circulation

Blood Flow: Lungs Blood flow in the pulmonary circulation is unusual in that: The pathway is short Arteries/arterioles are more like veins/venules (thin-walled, with large lumens) They have a much lower arterial pressure (24/8 mm Hg versus 120/80 mm Hg) The autoregulatory mechanism is exactly opposite of that in most tissues Low oxygen levels cause vasoconstriction; high levels promote vasodilation This allows for proper oxygen loading in the lungs

Effect of Po2 on Blood Flow Blood Flow % Control Alveolar PO2 24

Distribution of Blood Flow Bottom Top 25

Hydrostatic Effects on Blood Flow Ppc = capillary pressure PALV = alveolar pressure Distance Flow 26

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